Phenol-formaldehyde (PF) resin was the first commercialized synthetic resin having wide application in coatings, adhesives, casting, engineered materials, household products, etc. The discovery of carcinogenic effects of formaldehyde (Zhang et al., 2009) and more stringent environmental regulations to reduce volatile organic compounds (VOCs) in recent years have created a need for improvement in PF manufacture, to reduce formaldehyde emissions during production, for example, or reduce off-gassing during final production stages.
Manufacturers are thus looking for greener and more environmentally friendly alternatives to conventional polymers such as PF. (Netravali and Chabba, 2003) Biomass is increasingly becoming a significant feedstock for fuels and chemicals. (Zakzeski et al., 2010) Together with cellulose and hemicelluloses, lignin constitutes one of the three main components of lignocellulosic biomass. Lignin is a polymer of three monomers: guaiacyl (G); syringyl; and p-hydroxyphenyl propane (p-H)-type. (Tejado et al., 2007) Among the components of its structure, the phenolic group of lignin is of particular interest and has attracted the interest of researchers as a substitute for crude oil based phenol in e.g., phenol-formaldehyde resins. (Effendi, Gerhauser and Bridgwater, 2008)
Phenol has been replaced with lignin and cardanol, for example, but formaldehyde continues to arouse environmental concerns and its exposure levels are regulated in the United States. (Kowatsch, 2010; Hahnenstein et al., 1994) One approach has been to replace PF resins with more expensive resins (Kurple, 1989) despite relatively poor economics.
There has been extensive effort towards using lignin as an alternative to phenol in synthesizing lignin-modified phenol-formaldehyde (LPF) resins, but incorporating lignin directly into the PF synthesis has been a challenge as crude lignin has fewer reactive sites than phenol to react with aldehydes. (Wang et al., 2009) Lignin modification to obtain more reactive functional groups has been commonly practiced to this end, which includes phenolation, (Alonso et al., 2005) methylolation (Alonso et al., 2004), demethylation (Ferhan et al., 2013) and hydrothermal de-polymerization/liquefaction (Cheng et al., 2012; Cheng et al., 2013). The final resin behavior was found to be very dependent on the chemical and physical properties of the lignin. Direct phenolation of lignin due to its simplicity was widely applied for use in phenolic resins. (Alonso et al., 2005)
Glucose is the main building block of cellulose, hemicellulose, and starch, and is the most abundant renewable fixed carbon source in nature. With the projected depletion of fossil resources approaching, glucose could be a future carbon source for fuels (bio-ethanol and bio-butanol, dimethyl furan, etc.) and other chemicals after certain chemical transformations. The transformation of glucose to HMF, a platform chemical, has been demonstrated in water, organic solvents, chloride salts, and ionic liquids. (Yan et al., 2009; Zhao et al., 2007; Li, et al., 2009; Binder and Raines, 2009)
Hexamethylene tetraamine (HMTA), a condensation product of ammonia and formaldehyde is commonly used for curing of novolac-type phenolic resins. Use of HMTA is also restricted due to its decomposition to form ammonia and formaldehyde in curing and applications. (Nielsen et al., 1979; Richmond, et al., 1948) Methylene bridges form between phenolic benzene rings during novolac synthesis with at least one ortho- or para-position remaining on the phenol rings of novolac resin, the general structure of novolac being (Knop and Pilato, 1985):
There is the possibility of forming additional methylene bridges by using higher temperatures, but application of this approach is limited by HMTA being a hazardous air pollutant. (Lytle, Bertsch and McKinley, 1998) Thus exploring green harders as substitutes for HMTA for novolac resin curing has also received growing research interest.
Simitzis et al. (1996) produced novolac-type resins cured with mixture of HMTA and one of the following components: the residue from pressing olives and separation of oil, Kraft lignin (KL), hydroxymethylated Kraft lignin (KLH), and cellulose (CEL). The activation energy (Ea) and pre-exponential constant (k) of the curing reactions were found to be HMTA<HMTA/KLH<biomass<KL<CEL. It was indicated that although Ea and k vary with different curing agents, the reaction order, n, was practically the same (n=1). However, the mechanism of cross-linking with these new curing agents was not proposed. (Simitzis et al., 1996)
2,6-di(hydroxymethyl)-p-cresol (a), 3,3′,5,5′-tetra(hydroxymethyl)-4,4′-isopropylidenediphenol (b), and 2,6-bis(2-hydroxy-3-hydroxymethyl-5-methylbenzyl)-4-methylphenol (c), with the following structures,
have been used as curing agents for novolac resins. Hard polymers with higher physicomechanical characteristics compared with those cured with HMTA were obtained. (Sergeev et al., 1995) Lignin includes methoxylated phenylpropane structures (Zakzeski et al., 2010) structurally similar to the above curing agents, hence shall follow similar cross-linking mechnism as the above curing agents, when used as an HMTA replacement for curing of novolac PF resins. (Grenier-Loustalot et al., 1996)
Organosolv lignin (OL), which is obtained by treatment of wood or bagasse with various organic solvents, is typically low in sulfur content and of high purity. (Sarkanen et al., 1981) OL is known to be produced commercially as a by-product from cellulosic ethanol processes. Kraft lignin (KL) is a by-product of Kraft chemical pulping of lignocellulosic materials, in which high pHs and considerable amounts of aqueous sodium hydroxide and sodium sulfide are employed at temperatures between 423-453 K for about 2 h to dissolve lignin. KL is produced in large quantities (approx. 70 million tons per year), but is currently used mainly as a low-value fuel in recovery boilers at pulp/paper mills for heat/power generation.
Fiber reinforced composite (FRC) using of PF resin (novolac) as a polymer matrix is a typical application of novolac PF resins. Owing to its high strength, high stiffness and good corrosion resistance, FRC using PF resin has gained popularity in windmill blades, boat, aerospace, automotive, civil infrastructure, sports as well as recreational products. Bio-composites produced with cost competitive green components are more promising. Considerable growth has been seen in the use of bio-composites, such as glass fibers reinforced composites with bio-based polymer matrix materials, in the automotive and decking markets over the past few decades. (Shibata et al., 2008; Suharty et al., 2008)
The most commonly used curing agent for PF novolac is still hexamethylenetetramine (HMTA). Curing conditions, reaction mechanism and kinetic parameters between PF novolac and HMTA or paraformaldehyde have attracted lots of research interests. For instance, fast quantitative 13C NMR spectroscopy was applied to characterize the degree of polymerization, number average molecular weight, and the number of un-reacted ortho- and para phenol ring. (Ottenbourgs et al., 1995) The curing behavior of novolac resin and paraformaldehyde was discussed by using solid-state 13C NMR. (Ottenbourgs et al., 1998; Bryson et al., 1983) This technique showed that the formaldehyde/phenol ratio and the degree of the curing conversion can be quantitatively determined. However, it was found that paraformaldehyde was unable to completely cure the novolac. Zhang et al. also investigated the chemistry of novolac resin and HMTA upon curing using 13C and 15N NMR techniques. (Zhang et al., 1997; Zhang et al, 1998; Lim et al., 1999) Special attention was given to benzylamines and benzoxazine that were formed as the reaction intermediates during the curing process. Methylene linkages are formed to link novolac molecules with para-para linkages at lower temperatures, while they are thermally less stable than ortho-linked intermediates.
Curing parameter and conditions are critical to properties of phenolic materials. One of the most common analyses was performed by differential scanning calorimetry (DSC). The activation energy of approximately 144 kJ/mol and reaction constant have been reported. (De Medeiros et al., 2003) Their curing reaction, recorded by rheometrics mechanical spectroscopy, was described by a self-acceleration effect and a third order phenomenological equation. (Markovic et al., 2001) Wan et al. further evaluated effects of the molecular weight and molecular weight distribution on cure kinetics and thermal, rheological and mechanical properties of novolac harden by HMTA. (Wan et al., 2011) They reported that the novolac resin with a lower molecular weight exhibited higher reaction heat and reactivity, faster decomposition rate upon heating, lower char residue at 850° C. and the composite materials presented higher flexural strength.